A wide variety of forest products are manufactured from wood fibers. The present invention focuses upon a class of wood-fiber products that are molded in three dimensions under conditions of heat and pressure to produce a structural wood fiber web that serves as the principal structural component of composite structural-fiberboard panels. The geometry of the web of the present invention permits the use of straightforward mass-production techniques, utilizing a simple rigid mold that may be pressed together with one-dimensional forces. When the fiber web is bonded to sheet coverings or facings to produce a composite panel product, the composite structure forms a strong, lightweight, rigid three-dimensional truss. The prior art does not disclose a wood-fiber structure of the form of the invention nor does the prior art show fiberboard structures having three-dimensional features that may be so readily mass-produced in a wide range of overall board thickness.
In the prior art, methods and apparatus are disclosed for forming various other fiberboard products having three-dimensional elements. For example, Setterholm and Hunt in U.S. Pat. No. 4,702,870 describe a method and apparatus for forming three-dimensional structural components from wood fiber. Their method and apparatus require the use of a resilient mold insert to form three-dimensional features in the finished fiberboard product. The resilient mold insert is most commonly composed of an array of elastomeric protuberances. The elastomers are attached to a rigid support plate.
Elastomers are weak and difficult to attach firmly to the support plate. In mass-production of wood-fiber products, elastomeric mold elements exhibit problems with compression-set and relatively rapid deterioration under the heat and pressure necessary for product consolidation and drying. As a result, the elastomeric mold elements have a relatively short lifetime and need to be frequently replaced in high-speed production facilities. In addition to short mold lifetimes, the three-dimensional fiberboard objects disclosed in the invention of Setterholm and Hunt are limited to objects having a flat face, backed by webs extending approximately normal to the flat face.
Heat transfer from the resilient mold insert of Setterholm and Hunt to the fiber mat is slow because of the low thermal conductivity of the elastomeric elements of the mold inert and because of long thermal-conduction pathways to regions of the fiber between the elastomeric mold elements. Slow heat transfer results in long drying times within the press, a major problem for this method, particularly for thick products. Drying speed may be increased using radiowave heating of the fiber mat, but this increases the complexity and cost of equipment used to form and dry the fiberboard products.
Thus, the invention of Setterholm and Hunt reveals the structure of a very specific wood fiber product that is formed using a method and an apparatus that are not readily adapted to high-speed mass-production, particularly in the case of thick panel products. As will become apparent in the next several sections, the present invention defines a new fiber structure that may be used in many of the same applications as the invention of Setterholm and Hunt, yet without the drawbacks in product formation and mass-production encountered with the invention of Setterholm and Hunt.
A process for making grids from fibers, described by Hunt in U.S. Pat. No. 5,277,854, also uses the idea of a resilient mold insert which is capable of forming objects in three-dimensions. Because of the use of a resilient mold insert, this invention suffers from the same difficulties as does the invention of Setterholm and Hunt. In addition, while the mold insert of Hunt is capable of generating three-dimensional forces, it is used to generate a fiber product that has generally two-dimensional features only.
In U.S. Pat. Nos. 5,198,236 and 5,314,654, Gunderson and Gleisner describe a method and apparatus that uses a rigid mold to form three-dimensional features in structural fiberboard products. Once again, the fiberboard products disclosed in their patent are limited to flat-faced objects backed by webs extending approximately normal to the flat face. In addition, the rigid mold elements disclosed by Gunderson and Gleisner must be retracted during consolidation of the fiber. In U.S. Pat. No. 5,314,654, a second forming step is required using a resilient mold insert similar to that of Setterholm and Hunt. Therefore, formation of the structural fiberboard product disclosed by Gunderson and Gleisner suffers from the same difficulties as have been pointed out for the invention of Setterholm and Hunt. In addition, the need for retractable mold elements makes this method complex and expensive.
Prior art disclosed in U.S. Pat. No. 5,316,828, by Miller, reveals a reinforced fluted medium and corrugated fiberboard that has increased strength and stiffness in comparison to conventional corrugated fiberboard due to the addition of three-dimensional elements in a simple corrugated fiberboard structure. The three-dimensional elements take the form of adhesive material applied along lines that are transverse to the flutes. The adhesive at least partially fills in and bridges across the valleys of the flutes, holding the corrugated board more rigid under compressive and bending stresses both along the corrugations and across the corrugations.
The invention requires two distinct materials, wood fiber and adhesive, to form the basic structure of the product. The structure of Miller is therefore not formed as a single piece and would require multiple manufacturing steps. In addition, considerable adhesive would be required to fill in the valleys to the top of the flutes. The adhesive could fill in and bridge only a small portion of the flutes in thick corrugated boards, making the technique ineffective for thick corrugated panels. Finally, application of adhesive to both sides of the fluted medium would increase product weight and material cost, and complicate board manufacture.
In U.S. Pat. No. 4,726,863, Cline describes a method for making a high-strength composite paperboard panel. The panel is composed of an undulated midstratum layer to which are adhesively bonded an underlayer and an overlayer. There is no variation of the structure along the flutes formed by the undulations, making the structure generally two-dimensional and placing it in a different structural class than the present invention. Because of its two-dimensional structure, which is similar to the structure of conventional corrugated boards, the panel product disclosed by Cline has less strength and stiffness across the undulations compared to along the undulations.
In summary, numerous composite wood-fiber panel products are described in the prior art. Only a few of these products are comprised of three-dimensional elements which produce fiberboard panels having high strength-to-weight ratios and approximately equal strength and stiffness in all directions within the plane of the panels. The prior art disclosures of three-dimensional elements in fiberboard panels all suffer from significant difficulties in production of thick panels and in mass-production at high-speeds. These difficulties have impeded implementation of much of the prior art by the fiberboard industry and end-users. The present invention overcomes these difficulties by defining a new three-dimensional wood-fiber structure that has excellent strength-to-weight properties, and yet it can be readily mass-produced in the form of both thin and thick panels.